The present invention relates to a method for acknowledging receipt of data or the like in multiplex transmission, and more particularly, to a receipt acknowledgement method for returning an acknowledgement signal with a NRZ (non-return-to-zero) code, to thereby efficiently transmit data in multiplex transmission of a type using carrier sense multiple access with collision detection.
In a network constituted by a plurality of multiplex nodes connected to a common multiplex transmission line, for instance, data transmission is carried out among these nodes. When the nodes are each equipped with a computer for communication and operable independently from one another, these nodes can transmit data to the common transmission line with respective arbitrary timing, so that collision of data can occur on the transmission line.
In order to arbitrate collision of data, a method of carrier sense multiple access with collision detection (CSMA/CD), for example, is applied to the aforementioned type of networks. With the CSMA/CD method, a transmitting node, as one of the plurality of nodes, detects whether the other nodes are using a transmission line or not on the basis of the presence/absence of a carrier, and starts data transmission if the transmission line is not being used. After the start of data transmission, the transmitting node continues to monitor the presence/absence of a data collision on the transmission line. When a collision occurs, the transmitting node interrupts the data transmission, and transmits a jam signal for bringing data transmission by the other nodes into the standby state. More preferably, data transmission is carried out with the aid of CSMA/CD of a non destructive arbitration type. The CSMA/CD of this type is designed such that a dominant code appears on the transmission line when the dominant code collides with a recessive code, and that priority of data is set in advance by using a combination of dominant codes and recessive codes. Upon occurrence of collision of data whose priorities differ from each other, transmission of data with low priority is automatically interrupted, and data with high priority, which takes precedence in transmission, is transmitted, while being prevented from data destruction.
Also known is to transmit data frame by frame, while checking whether or not the thus transmitted data has been received normally. Typically, return of an acknowledgement signal (ACK signal) is requested each time transmission of a data frame is finished, and a judgment is conducted in accordance with the presence/absence of return of the acknowledgement signal, to check whether or not receipt of the data has been made normally. To this end, data frames are each provided with an ACK signal area. More specifically, an ACK signal return area is provided in such a manner that it follows a data frame transmission area along the time axis of data transmission. Each time the ACK signal return area is entered, an ACK signal is returned from each of the nodes which have normally completed receipt of data frame with a timing assigned to the node. Then, each of the ACK signals is stored at a corresponding address position in an ACK signal storage area constituted by, e.g., a buffer memory to which the transmitting node can refer.
Typically, a PWM (pulse width modulation) signal consisting of one logical bit divided into three bit sections (hereinafter referred to as "phase") is used as an ACK signal. As for the PWM signal, first and third phases are fixed at an active signal level and at a passive signal level, respectively, while a second phase takes an active or passive signal level, to thereby represent a state of logic "0" or logic "1".
Specifically, at the first phase of each bit of the ACK signal return area, an active signal, for example, is sent out from a transmitting node onto a multiple bus. Each of the nodes which have normally completed data receipt makes the multiple bus active at the same time as it detects a leading edge of the first phase of the bit area, assigned to the node, of the ACK signal area, and keeps the active state of the multiple bus till the second phase of the above bit area, to thereby make the associated bit show the logic "0" representative of normal receipt. In the meantime, when data is not normally received by a receiving node because of a local error on a network, etc., the above receiving node does not make the multiple bus active even if the leading edge of the first phase of the associated bit area is detected, whereby the second phase of the above bit area is brought into the passive state, so that the above bit shows the logic "1" representative of defective receipt.
In this way, when an ACK signal is not returned from a receiving node, the transmitting node judges an occurrence of abnormality, and resends data. Data is resent, for example, three times at the most till all the receiving nodes registered on ACK management return the ACK signal. And the receiving node which does not return the ACK signal even after data is resent three times is regarded as failure, and deleted from registration. In the meantime, when an ACK signal is newly returned after the local error is solved, the node corresponding to the ACK signal is newly added to the registration.
As mentioned above, when the ACK signal area is constituted with the PWM coding method, a required frequency bandwidth is tripled provided that a data transmission speed is the same, compared with the case where the NRZ (non-return-to-zero) coding method is applied. Thus, with the PWM coding method, radio interference such as spurious radiation can easily occur. Therefore, an ACK signal can not be returned at high speed, and a difficulty such that data transmission can not be sped up is encountered. Also, when the NRZ coding method is simply applied to an ACK signal area for speedup of data transmission, as clock frequency is generally different between multiple nodes in the strict sense, it is hard to take bit synchronization to make each ACK signal correspond to an associated one multiple node.